1. Field of the Invention
The present invention relates to a two-way multiple access communication system comprising at least one central station, a plurality of user stations, and a transmission network for exchanging at least video information comprising user information in sessions between the central station and the user stations, and for exchanging session accompanying control information. Such a network can be a CATV-network, a radio network, or the like.
The present invention further relates to a central station and a user station for use in such a system.
2. Description of the Related art
A two-way multiple access communication system of this kind is known from the European Patent Application No. 0 140 475, corresponding to U.S. Pat. No. 4,633,462. An interactive multiple access communication system is described in which, in a downstream direction of a CATV (Community or Cable Television) network, as a transmission network, a plurality of television programs are supplied by a CATV head end, as a central station, to a plurality of user stations, the transmission network being a coaxial cable. For offering interactive television services such as video-on-demand, response to multiple choice queries, or the like, in U.S. Pat. No. 4,633,462 common forward and reverse channels are provided which are common to all user stations, and which are separate from television channels for providing the television programs, the reverse channel occupying a frequency band below 30 MHz, and the forward channel occupying a TV channel equivalent frequency band above 50 MHz. Interactivity characterizes a system in that a maximum response time from the central station to a subscriber station input is less than a predetermined response time, e.g., less than one second. Via the forward and the reverse channels, session accompanying control information is exchanged between the central station and the user station in traffic packets containing user station addresses inter alia. For solving a packet collision problem on a common control channel, typical for CATV systems having many branches and sub-branches, in U.S. Pat. No. 4,633,462 a retransmission mechanism is disclosed in which the central station acknowledges control messages from the user station. Although the known system operates satisfactorily to some extent in providing interactive television inter alia, capacity problems arise when many user stations having a lot of reverse traffic will have to communicate with the central station simultaneously. Then, either the system becomes blocked, or a true interactive service can no longer be provided. Furthermore, when applying a common control channel, the channel has to be a relatively wide-band channel with relatively high speed transmission. This would disadvantageously lead to the user station needing a relatively high-speed modem for communicating control messages, such a modem being relatively expensive.